The present invention relates to a method and a plant for preparing mineralised Portland cement clinker in a dry or semidry process kiln system where the rawmix feedstock is being subjected to preheating, calcination, burning, and, finally, cooling.
The major oxide components of Portland cement clinker (CaO, SiO2, Al2O3 and Fe2O3) normally account for 96-97% of the chemical analysis. The relative levels of these four oxides control the proportions of the four principal clinker minerals, C3S, C2S, C3A and C4AF, and these proportions have a fundamental influence on cement properties. However, the minor components which account for the remaining 3-4% can have a very significant influence on the clinker-making process and the properties of the cement.
In the pure four component system C3S does not form below 1250°C. The presence of minor components can lower this temperature, thus facilitating C3S formation. The term mineraliser is used for components which encourage the formation of C3S and facilitate the sintering reactions in the rotary kiln.
A method for manufacturing mineralised clinker is known from the British patent No. 1 498 057. According to this method fluorine and sulphur are added during the rawmix preparation, usually in the form of fluorite (CaF2) and gypsum (CaSO4 ·2H2O), the major goal being to achieve a clinker with a fluorine content of about 0.25 wt% and a SO3 content of about 2.5 wt%. The method of said patent focuses on utilization in a rotary kiln of the wet process type, which was extensively used at the time of publication in 1975, but regarded today as being uneconomic due to the substantial amounts of energy needed for drying, and it has proven to be extremely difficult to transfer the method to the newer, more energy efficient dry or semidry process kiln systems incorporating a preheater and a calciner.
So, despite the very substantial improvements in the cement quality which can be achieved (see e.g. Moir, Phil Trans Roy Soc Lond., 1983, A310, 127-138) by this method according to the British patent No. 1 498 057, it is noteworthy that the field of application of this method has been very limited. This is probably ascribable to the operational difficulties associated with the implementation of the method, both in terms of controlling of clinker production and with respect to the occurrence of blockages in cyclones and riser ducts.
Therefore, it is the objective of the present invention to provide a method and a plant for manufacturing mineralised cement clinker, where the plant is constructed so that preheating and at least part of the calcination process takes place outside the kiln, in such a way problems associated with the known technique are avoided.
A first operational difficulty is to determine how to control the input of mineraliser and how, at the same time, to ensure the necessary degree of homogeneity in the rawmix feedstock. From the British patent No. 1 498 057 it appears that the mineraliser is mixed with the rawmix feedstock before it is burned (page 5, line 89 - page 6, line 43).
Addition of the mineraliser in this way will, admittedly, result in a high degree of homogeneity, but it will not be possible to make a swift regulation of the mineraliser input relative to the raw materials so that the kiln operation and the properties of the finished cement clinker can be controlled in this manner, since the mineraliser constitutes a fixed percentage of the total quantity of raw materials.
Particularly in connection with the production of mineralised clinker with a high content of sulphate, it is of importance to control temperature variations in the burning zone. This is due to the tendency of the sulphate to decompose into SO2 in the burning zone and to condense in the cooler regions of the kiln. When the burning zone temperature is increased, the evaporation of SO2 will increase, which will result in higher concentrations of sulphates in the cooler regions of the kiln. The situation may become so critical that continued kiln operation becomes impossible because of ring formations in the kiln or the formation of build-ups or blockages in the preheater system unless the input of sulphate to the rawmix feedstock is temporarily reduced or eliminated altogether.
Conversely, if the burning zone temperature has decreased, a severe dust circulation between kiln and cooler may result and continued kiln operation becomes impossible unless the sulphate input is reduced or eliminated.
DE-A-1 646 647 discloses a method for preforming mineralised cement in a kiln system wherein the rawmix stock is subjected to preheating, burning and cooling and mineraliser is fed to the feedstock stream.
By using the method according to the invention, as indicated in claim 1, these problems are avoided since it is possible to use the amount of mineraliser input as a controllable parameter for the manufacture of mineralised clinker during off-set kiln operating conditions.
If appropriate, some of the mineraliser can be added to the rawmix feedstock prior to grinding and homogenization, whilst the remainder of the necessary mineraliser input is used for controlling the process. In some cases, the rawmix feedstock will have a natural content of mineraliser, but, provided that the content is not excessively high, it will still be possible to control the overall process by further addition of mineraliser. It is also possible to add two or more different mineralisers to different points in the process line, independent of each other, in order to control the process and the product.
A second operational difficulty which is well-known when burning ordinary Portland cement clinker with a high content of mineraliser, particularly SO3 and F, in the rawmix feedstock, relates to the precipitation of solids and the occurrence of blockages when the material passes through a temperature range of about 700-900°C.
It has been found that in the presence of significant concentrations of these mineralisers in the preheating zone, the condensation of chlorides on the preheater feedstock particles results in the formation of a minor, but significant, quantity of a melt phase at temperatures as low as 680°C which, upon reaction with the oxide components in the feedstock at the high CO2 partial pressures prevailing in the suspension preheater cyclones, solidifies through the formation of the mineral spurrite (2C2S·CaCO3).
The formation of in excess of 5 wt% spurrite in the feedstock can result in impaired flow properties of the feedstock leading to severe build-ups and ultimately to production shutdown.
In US patent No. 5.183.506 the method chosen to solve this problem involves the use of a special mixture of gypsum.
In order to reduce or eradicate the problems associated with build-ups in the preheater cyclones and the riser ducts between the cyclones, the mineraliser may advantageously be added at a stage where calcination has begun to take place, i.e. where the feedstock has been preheated to more than 700-750°C, preferably more than 800°C.
Such action will further enhance production in ensuring a smooth flow of the feedstock through the preheater, since a substantial deterioration of the flow properties of the feedstock may occur when the mineraliser is added, depending on the type of mineraliser being selected.
In a precalcining plant with a separate cooler, as for example a grate cooler, it may further be advantageous to add the mineraliser to the tertiary air duct, through which heated air is directed from the cooler to the calciner. The air in the tertiary air duct will, in connection with this solution, ensure that the mineraliser is conveyed to the calciner.
If appropriate, the mineraliser may be subjected to prior drying in excess air from the cooler, before it is added to the tertiary air duct.
The mineraliser can be a sulphurous product, and in conjunction with this product another mineralising component or an auxiliary substance containing fluorine or copper or zinc oxide may be used, but frequently, addition of such a component or substance will not be necessary since it is already present in the raw materials.
It has been recognized that through the combined presence of fluorine and sulphur components in the clinker at concentrations of at least 0,15 wt% F and 1,5 wt% SO3, both early combination in the burning process and enhanced strength development of the finished product is achieved.
Through the reduction in the contents of the mineraliser components in the rawmix feedstock and through the introduction of the said components to the feedstock stream at a temperature greater than 800°C, the mineraliser content in the feedstock in the preheating zone is reduced, thereby effectively eliminating or at least significantly reducing the risk of blockages in the preheating zone, which is important when the mineraliser is a sulphur containing component. The exact degree of reduction in the mineraliser content in the feedstock in the preheating zone will obviously depend on the degree of recirculation characteristic of the kiln system in question, although it is clear that the said content can never be lower than the corresponding content in the rawmix feedstock. However, investigations have indicated that if the risk of blockages is to be eliminated or substantially reduced, contents of at the most 1,2 wt% SO3 on a loss on ignition free basis or 0,14 wt% F on a loss on ignition free basis or both in the feedstock in the preheating zone should be observed and the same limits should therefor apply to the composition of the rawmix feedstock.
The sulphur containing component may inter alia be natural gypsum (CaSO4·2H2O), hemihydrate (CaSO4·½H2O)(bassanite), anhydrite (CaSO4), Ca or Mg langbeinite, barytes or other sulphur containing minerals also containing calcium or magnesium. The sulphur containing component could also be a waste product e.g. FGD (Flue Gas Desulphurization) gypsum, sulphite or sulphide containing desulphurization products, sulphate containing industrial byproducts such as ammonium or iron-based sulphates, cement kiln dust, sulphur-containing fuel such as petcoke or high sulphur coal or oil.
The desulphurization products are formed when flue gases, for example from power plants, are cleaned of SO2. Semi-dry flue gas desulphurization produces a by-product of calcium sulphite and calcium sulphate, which may also contain flyash and other constituents. Modern, wet-process desulphurization methods are capable of producing a very clean gypsum, emerging initially, however, in the form of wet sludge or a filter cake.
After being subjected to drying, the last-mentioned product may, for example, find application in gypsum wallboard manufacturing or it may replace natural gypsum in the manufacture of cement where it is inter-ground with the clinker in the cement mill. At the present point in time, the application range for the desulphurization product, derived from semi-dry desulphurization, is very limited, and it is further a complicating factor that disposal of the product is difficult due to the unfavourable rheological properties of the product.
Another interesting by-product is anhydrite contaminated with CaF2 and H2SO4 formed during the manufacture of hydrofluoric acid from fluorspar and sulphuric acid.
In the future, another by-product may emerge which involves the destruction of asbestos: one method proposed entails the dissolution of asbestos in hydrofluoric acid (HF) followed by neutralizion with burnt lime, whereby a product consisting of MgO, SiO2, CaF2 and a certain amount of CaSO4·2H2O results. Finally, another option to be considered is the use of phospho-gypsum (i.e. the gypsum which is the by-product derived from the manufacture of phosphoric acid on the basis of raw phosphate and sulphuric acid).
By means of the method according to the invention it is now possible to employ these waste products in the cement manufacturing process in an appropriate manner, since experience has shown that the operating difficulties discussed above may be avoided when the waste products are not processed together with the raw meal, but rather added separately to the process. Examples of suitable fluorine containing components are fluorite, fluor apatite, cryolite, or industrial byproducts containing fluorine such as hexafluorosilicic acid or silicontetrafluoride, preferably fluorite.
Broad aspects of the invention are also defined in claims 8 and 13.
The invention will now be explained in further detail with reference being made to the drawing which shows an embodiment of the invention.
The drawing shows a cement kiln plant of the generally known type which comprises a preheater 1, which in the figure consists of three cyclones, a calciner 2 with a separation cyclone 3 and a rotary kiln 4 with clinker cooler 5.
In a plant of the aforementioned kind, manufacturing of cement is carried out according to a method where cement raw meal is introduced at the inlet 13 at the top of the preheater 1, passing through the preheater in counter-current with the exhaust gases from the kiln 4 and the calciner 2. The exhaust gases are drawn up through the preheater 1 and discharged at 14 by means of a not shown exhaust gas fan. The raw meal is conveyed from the preheater 1 via a duct 7 down into the calciner 2 where it is calcined and passed on in suspension to the separation cyclone 3. Here the calcined raw meal is separated from the exhaust gases and conveyed by means of a duct 6 to the rotary kiln 4 where, by means of heat from the burner 9, the rawmeal is burned into clinker. The clinker then drops into the cooler 5 where it is air-cooled. One part of the hot air thus produced passes to the rotary kiln, another part is directed through the tertiary air duct to the calciner. The remainder of the cooling air drawn into the cooler is diverted at 10.
A mineraliser which does not have any detrimental effect upon the flow properties of the raw meal, may, in principle, be added at any location after the raw meal has left the store where the raw meal can have been subjected to homogenization, and after the raw meal has been added to the process through a dosale apparatus.
A mineraliser, such as a sulphur containing component emerging for example as a by-product of flue gas desulphurization, may advantageously be fed to the calciner either directly via the duct 12 or by adding the mineraliser to the tertiary air duct 8 via the duct 11 so that the mineraliser is carried by the hot air from the cooler to the calciner. Thus, when the mineraliser is directed into the calciner and into the subsequent separation cyclone, the calciner and the cyclone will operate as an homogeniser, facilitating thorough mixing of SO3 in the raw meal stream.
If the by-product contains sulphur in a lower state of oxidation, e.g. sulphite or sulphide, it may advantageously be added at 11 or 12 so that these sulphur compounds can be oxidated into sulphate in the calciner. If added at 13, this will result in the formation of a small amount of SO2 being entrained in the exhaust gases. If drying of the mineraliser is required prior to use, it will be possible to extract hot excess air from the cooler at 10 for use in a conventional drying apparatus.
In a preferred embodiment of the method where both sulphur and fluorine might be present, the composition of the rawmix feedstock is such that the sulphur content XS is at the most 1,0 % by weight calculated as SO3 on a LOI (Loss On Ignition) free basis, or the fluorine content XF is at the most 0,12 % by weight calculated on a LOI free basis, or both of these conditions are fulfilled, in particular that XS is at the most 0,8 wt%, or XF is at the most 0,10 wt%, or both, especially that XS is at the most 0,6 wt%, or X, is at the most 0,08 wt%, or both, such as that XS is at the most 0,4 wt%, or XF is at the most 0,06 wt%, or both.